Modern computer networks are designed in a highly structured way. To reduce their design complexity, the networks are organized as a series of layers, each one built upon its predecessor.
The OSI reference model has seven layers whose functions are defined according to the international standardized protocols. The first layer which is the physical-layer, is concerned with transmitting raw bits Over a communication channel. Whereas the second layer which is the data link layer, is to take a raw transmission facility and transform it into a line that appears free of transmission errors to the network layer. To accomplish this goal, the data are broken up into frames, each of which is transmitted as many times as necessary to ensure that it has been received correctly. To prevent a fast sender from overrunning a slow receiver, the data link protocol always provides for flow control.
The environment in which the present invention can be implemented is described in detail in EP patent application A 365 731. In such an environment the subject invention deals with a method for defining a reliable and efficient exchange of data at the level of data link layers between two machines physically connected with a coaxial cable (for example the main central control unit comprising a shared memory and an adapter attached to different users). Therefore, the logical protocol has to establish an efficient exchange of data and control flow between the shared memory subsystem and the communication adapters.
The constraints given by the environment are multiple and known by the man skilled in the art. As each communication adapter may house one or several logical users, the flow of data for each user must be easily differentiated on the interface which is between the adapters and a depository storage of the central control unit. Multiple adapters must be able to share the same physical interface which can be named the manager of storage.
The most important constraint is that the adapters must be the initiators of all data exchanges, for the reception as well as for the transmission. This particular type of protocol springs from the notion of slave and master, where all the adapters are considered as masters and the shared memory as a slave.
Besides, in order to avoid the obstruction of the data transmissions, the exchanges of data and control messages must be flow-controlled in both ways from the shared memory to the adapters as well as from the adapters to the shared memory.
Therefore, the main object of the present invention is to provide protocols for exchanging messages between communication adapters and shared memory subsystem comprising a depository storage, a manager of storage and a microprocessor according an efficient data link protocol.